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primops: make nature of foldl' strictness clearer

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* Clarify the documentation of foldl': That the arguments are forced
  before application (?) of `op` is necessarily true. What is important
  to stress is that we force every application of `op`, even when the
  value turns out to be unused.

* Move the example before the comment about strictness to make it less
  confusing: It is a general example and doesn't really showcase anything
  about foldl' strictness.

* Add test cases which nail down aspects of foldl' strictness:
  * The initial accumulator value is not forced unconditionally.
  * Applications of op are forced.
  * The list elements are not forced unconditionally.
This commit is contained in:
sternenseemann 2022-10-16 01:32:55 +02:00
parent ac0fb38e8a
commit d0f2da214b
6 changed files with 25 additions and 3 deletions
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tests/lang/eval-okay-foldlStrict-lazy-initial-accumulator.nix Normal file
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# Checks that the nul value for the accumulator is not forced unconditionally.
# Some languages provide a foldl' that is strict in this argument, but Nix does not.
builtins.foldl'
(_: x: x)
(throw "This is never forced")
[ "but the results of applying op are" 42 ]